Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., below 100 mesh U.S. Standard screen size to about 5 microns.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistant heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion flame spray gun, the carrier gas for the powder can be one of the combustion gases, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon, and hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations.
The nature of the coating obtained by thermal spraying a metal or ceramic powder can be controlled by proper selection of the composition of the powder, control of the physical nature of the powder and the use of select flame spraying conditions. It is well known and common practice to thermal spray a simple mixture of ceramic powder and metal powder.
In the manufacture of gas turbines, abradable metal compositions have been available for thermal spraying onto the gas turbine parts for the purpose of reducing the clearance between the fan or compression blades and the housing. The blades seat themselves within the housing by abrading the coating.
Thermal sprayed oxides, such as zirconia, have been tried as abradable coatings for the higher temperature sections of turbine engines, but this has been done only with limited success. When such refractory oxides are thermal sprayed with sufficient heat, such as with a plasma spray gun, to provide a suitably bonded and coherent coating, the abradability of the coating is poor. It has also been found that the blade tips of turbines wear excessively. When an oxide is thermal sprayed under conditions of lower heat, many of the particles are not sufficiently melted and are trapped in the coating, thereby reducing the deposit efficiency. The resulting coatings have also been found to be friable and not sufficiently resistant to the erosive conditions of the high velocity gases and debris found in turbine engines.
U.S. Pat. No. 4,421,799 reflects progress toward a solution of these problems. A thermal spray powder is disclosed that is produced by cladding aluminum to a core of a refractory oxide material, specifically zirconium oxide, hafnium oxide, magnesium oxide, cerium oxide, yttrium oxide or combinations thereof. A binder is used, such as a conventional organic binder known in the prior art to be suitable for forming a coating on such a surface. Thermal spray coatings of such a powder are characterized by both abradability and erosion resistance and have been good prospects for use as abradable coatings in high temperature zones of turbine engines. However, further improvements have been deemed highly desirable.
U.S. Pat. No. 3,607,343 broadly discloses thermal spray powders having an oxide core such as alumina or zirconia clad with fluxing ceramic. A large number of fluxing ceramics are suggested that include high silicas. The thrust of the patent is the production of nonporous, wear-resistant coatings.
In view of the foregoing, it is a primary object of the present invention to provide an improved thermal spray powder for producing an abradable coating which is also erosion resistant.
It is a further object of this invention to provide an improved thermal sprayed abradable coating suitable for use in the high temperature portions of a gas turbine engine.